1. Field of the Invention
The invention relates to a method of fabricating an electrode used for a prismatic cell, and more particularly to a method of fabricating an electrode comprising a positive electrode in the form of a sheet, a negative electrode in the form of a sheet, and an insulating sheet sandwiched between the positive and negative electrodes, the electrodes and the insulating sheet being in wounded and flattened condition.
2. Description of the Related Art
As handy devices such as a note-type personal computer and a cellular phone has been recently fabricated in a smaller size and a lighter weight, a cell as a power source is required to be in a smaller size and a lighter weight accordingly. A cylindrical cell had been first used widely. However, a prismatic cell can be incorporated into a space in a handy device more efficiently than a cylindrical cell, when a cell or cells are to be incorporated into a handy device. Hence, a prismatic cell has been widely employed as a power source for a handy device.
For instance, Japanese Unexamined Patent Publication No. 6-96801 has suggested a method of fabricating an electrode used for a prismatic cell. According to the method, an electrode for a prismatic cell is formed by depositing a positive electrode and a negative electrode both in the form of a sheet with an insulating porous plastic sheet sandwiched therebetween, winding the deposition around a planar plate core, and pulling the core out of the wound deposition. In place of the planar plate core, there may be employed a core having a circular cross-section or a core having an oval cross-section. Before the thus formed electrode is inserted into a prismatic cell case, the electrode or wound deposition is compressed in a diameter-wise direction to thereby cause the electrode to have an ellipse cross-section in conformity with a shape of the prismatic cell case, as having been suggested in Japanese Unexamined Patent Publication No. 60501729 based on U.S. patent application Ser. No. 509133 filed on Jun. 29, 1983 which is further based on PCT/US84/00977.
However, a wound electrode formed by winding a deposition of sheets around a planar plate core or a core having a circular or oval cross-section often slips out of place or inwardly loosens in the vicinity of a corner at which the deposition is bent, when the deposition is compressed into an ellipse form. As a result, there is generated a gap between the positive and negative electrode sheets.
As a solution to this problem, Japanese Unexamined Patent Publications Nos. 8-153519 and 8-171917 have suggested a method including the steps of winding an electrode sheet deposition, which is comprised of a positive electrode in the form of a sheet, a negative electrode in the form of a sheet, and an insulating porous polypropylene sheet sandwiched between the positive and negative electrodes for electrical isolation therebetween, around a core having a rhombic cross-section a plurality of times, pulling the core out of the thus formed wound electrode sheet deposition, and compressing the wound electrode sheet deposition into an ellipse form. According to the Publications, it is possible to prevent the wound electrode sheet deposition from slipping out of place or loosening. The reason is as follows. That is, since the core having a rhombic cross-section has an adequate strength, an electrode sheet deposition does not bend while being wound around the core, resulting in that the wound electrode sheet deposition is not slipped out of place. Furthermore, the wound electrode sheet deposition is forced to have a bending tendency along an acute angle of the rhombic core. Hence, the wound electrode sheet deposition is compressed so that a portion having the bending tendency is made bent, ensuring that the wound electrode sheet deposition does not loose.
However, even after the thus formed wound electrode sheet deposition is incorporated into a prismatic cell case, there still exist a space located at the center of the wound electrode sheet deposition, a gap between the positive electrode sheet and the insulating plastic sheet, and a gap between the negative electrode sheet and the insulating plastic sheet. The reason why those space and gaps still exist is that the wound electrode sheet deposition is compressed as it is, namely, without any pretreatment.
Furthermore, the above-mentioned space and gaps keep a cell away from having an increased capacity.